1. Field of the Invention
The present invention relates to pressure sensors and devices for measuring displacement distances of pressure sensitive apparatus and particularly to those devices that utilize ultrasonic signalling.
2. Prior Art
Pressure sensors are widely known in the prior art and often employ a component that physically moves in response to pressure changes such as the movable contacts disclosed in U.S. Pat. No. 4,342,230; the movable condenser plates of capacitors disclosed in U.S. Pat. No. 2,025,719; and the movable ultrasonic reflector disclosed in U.S. Pat. No. 3,140,612. These devices suffer from the inaccuracies that result in using mechanical components such as leaf springs, helical springs or through the use of non-linear components such as variable capacitance.
Devices that specifically employ ultrasonic signal technology include U.S. Pat. No. 3,140,612 discussed above; a mercury column pressure gauge disclosed in U.S. Pat. No. 3,008,332 wherein ultrasonic sound waves propagate through columns of mercury and are reflected at a mercury-air interface for distance measurement; and apparatus utilizing expensive crystals disclosed in U.S. Pat. Nos. 2,527,208 and 4,479,070. Additional devices using ultrasonic distance measuring technology include the movable airborne ultrasonic transducers disclosed in U.S. Pat. Nos. 2,520,297 and 2,629,082.
In U.S. Pat. No. 2,985,018, a device and the associated electrical circuitry is provided for measuring the amplitude and frequency of vibrations of an ultrasonic signal-reflecting surface. A transmitter emits the sound waves that reflect off a surface vibrating at a frequency to be measured. The signal is reflected to a receiver. A reference signal path comprises a circuit which receives an oscillator signal from the oscillator that drives the transmitter. The reference signal is fed to a rotatable coil which can be manually turned during initial calibration of the device. This device operates on the principal of a phase comparison between the received signal and the reference signal. Movement of the reflecting surface will result in a phase difference between the two signals which can be interpreted as the distance of movement of the reflecting surface from some initial "zero" position established during initialization of the system. Such prior art device suffers from severe disadvantages. First, the coils used in the reference circuit are non-linear components and thus make calibration more difficult. The coils act as delay lines and they must be adjusted to provide that the reference signal travel time is compensated for the longer time it takes the transmitted signal to reach the reflecting surface and be received at the receiver. This is accomplished by rotating the reference coil to adjust the phase shift that occurs to provide no phase difference when the reflecting surface is at its "zero" position. Secondly, if the temperature changes within the measuring apparatus there is no ready approach to compensate for the different velocity of the sound waves at different temperatures and the resultant phase difference which is independent of the distance traveled by the sonic wave. Thirdly, reflections and echoes are a source of inaccuracies in enclosed chambers and the prior art does not disclose how the problem is to be solved. Finally, pressure sensors for use in systems, such as those in the heating, cooling and ventilating, require temperature compensation over a wide temperature range. None of the devices in the prior art disclose means or a method for dealing with this latter problem area.
In accord with this invention an improved pressure sensor is provided which is an inexpensive, simple device having great accuracy and an operating mechanism that maintains the accuracy over a long period of time without suffering degradation due to wear and tear. In addition, temperature compensation should be easy to achieve over a wide temperature range normally encountered in heating, cooling and ventilating systems. Also, non-linear components should be avoided for ease of calibration and circuit stability. Further, the improved device should be readily adaptable for use in measuring either static or dynamic air pressure in a duct. Finally, the pressure sensing device preferably should specifically eliminate reflections and echoes in and around the ultrasonic sound path and receiver transducers in order to ensure proper operation and enhanced high accuracy. None of the pressure sensing devices of the prior art meet all the above criteria in accord with the invention hereinafter described in greater detail.